Network Layer

1
Network Layer

• Provides services to Transport Layer
• Transparent services hide implementation, such
  as
• Topology, type of router
• Network addressing
• Issue of connectionless or connection-oriented
• Does it provide correct, sequenced delivery, flow
  control, negotiation for quality and cost of
  service, buffers?
• Note that IP is connectionless, and sits on top
  of other network protocols
• Internal organization of network layer
• Virtual circuits (assigned path for entire
  connection)
• Datagrams (individual routing of each message
  unit)

2
Datagrams versus Virtual circuits

• Connection establishment/ release on VC only
• Identification on each packet
• Virtual circuit uses short ID (assigned locally
  at each router)
• Datagrams use full source/destination addresses
• Each router of VC path must maintain state
  information
• Input port, circuit ID- output port, circuit ID
• If one router of path goes down, all work of its
  current connections is lost all routers must
  delete entries of shared routes
• Congestion control
• More difficult with datagrams, but discarding is
  easier

3
Routing

• Routing is determining a path from source to
  destination machine
• Forwarding is actually sending the packet
• Routing objectives frequently conflict
• maximize throughput, minimize response time,
  fairness within priority classes, stability,
  bounded waits, simplicity, correctness,
  robustness, security, predictability
• Nonadaptive and adaptive routing algorithms
• Hybrids (tables with multiple choices picked
  dynamically)
• Point to point, broadcast and multicast routing

4
Routing

• Build up tables to determine the output port for
  a specification destination
• Virtual circuits determine the route and set up
  tables during connection establishment
• Then all incoming packets are switched
  accordingly
• Tables contain
• Incoming portincoming VCI outgoing
  portoutgoing VCI
• VCIs are LOCAL each router changes them before
  forwarding the packet
• Datagram networks route each packet separately

5
Optimal (shortest or least cost) paths from graph
theory

• Sink tree optimum route given all weights
• Weights change difficult to determine globally
• Dijkstras algorithm (p. 366)
• Partition all nodes into three sets-
• Initially the source node - each iteration adds
  another node to this set
• Next to be considered (adjacent to all nodes on
  optimal route) need consider only those
  adjacent to last chosen node at each iteration
• All the rest
• Used for nonadaptive or adaptive routes based on
  metrics (hops, capacity, queue lengths, delays)

6
Nonadaptive (static) routing algorithms

• Flooding
• Must be dampened, else exponentially increases
  without end
• Useful for military and network control
  information
• Tables computed by network engineers based on
  topology, capacity, expected traffic, etc. and
  loaded into routers
• SNA uses this with multiple paths in table
  specific path is chosen based on parameters (such
  as security) determined dynamically
• Source routing, for example
• Poor responsiveness to node, link failure
• Not responsive to the addition of new nodes,
  links
• Poor responsiveness to change, specifically
  surges in traffic

7
Adaptive (dynamic) routing

• Distance vector routing
• (early Arpanet, currently in Cisco, IPX)
• Initially based only on queuing or transmission
  delays
• Routing table lists each router
• Outgoing line estimated total time to get there
• Routers obtain metrics from adjacent nodes
• Echo packets, queue measures, or hop count
• Routers update output port computations
• Distance vector sent perhaps every second to
  neighbors
• Cycles (count to infinity) can occur
• bad news travels slowly
• Oscillations- too much traffic is rerouted onto
  alternating paths

8
Routing Information Protocol (RIP)

• Uses distance vector routing between different
  networks
• Metric is hop (1 on each link)
• Nodes forward to neighbors a packet containing
  distance for each hop
• Limited to 15 hops

9
Adaptive Routing (continued)

• Link state routing
• Nodes find metrics for distance to each adjacent
  node
• Echo packet, etc.
• Metrics sent to ALL nodes on network (or part of
  it)
• Dijkstras used to compute shortest path to each
  router
• Packets sent periodically or only if there is a
  change
• (lots of packets just when there is congestion)
• Lost or out-of-order control packets
• Distributed with flooding with sequence s,
  timestamps
• Inconsistencies, cycles, oscillations are
  possible
• More robust than DV (only local info sent in DV)
• More overhead

10
Open Shortest Path First Protocol (OSPF)

• Implemented with Link-state algorithm with extra
  services
• Robustness
• May catch misconfigured node that advertises it
  can reach every node at cost of 0 (as happened
  with the San Diego IMP)
• Handles VPNs
• Authentication
• Cryptographic authentication added
• Hierarchies
• Assists scalability
• Domain is partitioned
• Necessary for RIP as well
• Load balancing (multi-path routing)

11
Home and foreign agents home and
care-of-addresses in Mobile IP

• Partition network into home locations with a home
  agent and foreign agent assigned to each location
  
• Each host is assigned a permanent home address
• Foreign Agent periodically broadcasts a packet
  advertising for new users
• New host either answers advertisement packet
  (simplifying media access) or may solicit
  advertisement
• New host registers with foreign agent, who
  registers with hosts home agent
• Sends to home agent a care-of address (usually
  the foreign agents IP address, but sometimes
  mobile host supplies the care-of address)
• Home agent now intercepts all packets for host
  (proxy ARP) and tunnels them to foreign agent
  returns foreign agents address to senders agent
• Sender sends subsequent packets to care of
  address (care of address is cached and
  periodically purged)
• Security issue of intruder impersonating foreign
  agent (send all Joes mail to me)

12
Routing for Mobile Hosts

• If a mobile host disconnects/ connects to a
  different network, its previous IP address cannot
  work.
• Mobiles listen for router advertisements to
  detect whether it has moved. May get DHCP to
  provide care of address () for routing and sends
  this to home agent.
• Its network address is unchanged
• Hosts want applications to continue without
  interruption, transparently

13
http//datatracker.ietf.org/doc/rfc3344

• IP Mobility Support for IPv4 (mobile IP)
• This document specifies protocol enhancements
  that allow transparent routing of IP datagrams to
  mobile nodes in the Internet. Each mobile node is
  always identified by its home address, regardless
  of its current point of attachment to the
  Internet. While situated away from its home, a
  mobile node is also associated with a care-of
  address, which provides information about its
  current point of attachment to the Internet. The
  protocol provides for registering the care-of
  address with a home agent. The home agent sends
  datagrams destined for the mobile node through a
  tunnel to the care-of address. After arriving at
  the end of the tunnel, each datagram is then
  delivered to the mobile node.

14
Hierarchical Routing

• Advantage - partition network so that control
  packets need not be sent to ALL routers tables
  need not contain ALL routers
• Example IP host address consists of a network
  number and host ID network is used to indicate
  country, and thus partition routing
• Disadvantage does not always find shortest path

15
Broadcast routing

• Broadcast media (broadcast routing is not needed)
• broadcast addressing is sufficient
• Point to point media
• Flooding with dampening
• Individual packet addressed to each node
• Spanning tree
• Packet copied at each router for each output line

16
Multicast routing

• Compare to VLANs
• Spanning tree
• Pruned according to group membership
• Routers must know group membership.

17
Routing in Peer-to-Peer Networks

• How to maintain a distributed database in a P2P
  network?
• Some information is stored on each machine
• One method is to maintain a logical ring
• Each node has successor address
• multiple successors for fault tolerance
• Predecessor to allow sending to both directions
• Node sends packet containing
• Its own address
• Key to identify needed information